The following is an idea I had that may be an easier to implement alternative than overhauling the resource system to allow for stockpiling. Taking inspiration from the changes mentioned in the new update previews, the idea is that there would be a delay from when the a tile is poached to when the extractor owner's resources actually become constrained. I envision the delay being determined by the distance from the poached trading node to the capital (i.e. 1 turn for every X territories crossed), which would reflect the real-world aspect of logistics. So if an enemy is at your capital poaching tiles, the effects would be felt immediately. However, if an enemy is poaching a node on a far off continent, IRL there should be some ships still en-route to the capital. I imagine this delay could affect both the attacker and defender, requiring a longer amount of time before poached tile resources become available based on the distance from the node to the attacker's capital.

Then if you are able to repel the looters before they gain access to your resources (i.e. trade caravans reach their capital), they should become immediately available as you are able to re-posses those trade caravans. However, if the enemy is able to gain access to your resources, there should be delay for those resources to become available again as new trade caravans would need to head to your capital from the trading node.

I see this system not only adding in a few turns of a stockpile, but also making WHERE you poach a trade route a lot more strategic of a decision. I.e. if you are already have sufficient access to a strategic resource, you may choose to poach the route closer to your opponent's capital to more immediately deny them access to that resource and "starve" them before your assault force arrives. However, if you're lacking in a resource, it will make a lot more sense to poach closer to your own capital.